Many types of magnetic strength measurement devices are known. Generally, they consist of an instrument with amplification and readout elements and a sensor connected to the instrument via a cable. Instruments with built-in sensors are generally used for large volume characterization of weak stray fields, and probes are generally used to measure stronger fields, for example, in coil bores or in narrow gaps between magnetic pole pieces.
A simple sensor of a magnetic field can comprise a coil of wire that responds to a change in magnetic field strength through the coil by outputting electrical power. In that example, a diode connected to a capacitor can collect and store energy from the coil, and a mechanical switch can transfer that energy to a lamp. Intensity of illumination of a lamp can provide an indication of magnetic field strength. In certain applications, however, multi-thousand turn coils are needed to sense particular types of magnetic fields.
Depending on the sensor type, the instrument incorporating the sensor may provide sensor excitation as a portion of the device. For example, a Hall effect sensor element can be excited by an adjustable current. Instruments may also utilize amplifiers to combat noise and, for example, to offset chopper amplification to nullify offset in the measurement of very weak fields. Other instruments have multidirectional measurement capability. For example, multiple Hall sensors can be set close together and orthogonal to each other to form a multidirectional probe.
In certain fields of use, however, general purpose magnetometers cannot reliably confirm the magnetic fields necessary for given applications. For example, in the field of magnetic degaussing, high strength and high frequency magnetic fields are sometimes applied to magnetic storage media, such as magnetic tape, computer hard drives, and the like, to erase any data stored thereon. To confirm that the media is being thoroughly erased, the strengths and directions of the applied magnetic fields must be confirmed. Although general purpose magnetometers may be adaptable to verifying the magnetic fields used in bulk degaussers, bulk degaussers generally apply magnetic fields within relatively contained spaces and provide media restraint or transport devices that interfere with the cable connected probes usually associated with high field strength sensing. Compact, non-wired survey units, however, generally lack high field strength measurement capabilities.
Much simpler known magnetic field detection or characterization devices used in checking fields created by bulk degaussers include visual indicators such as a variable number of blown fuse links. For example, visual indication devices may include self-adhesive thin films of high coercivity material patterned by the manufacturer, developed at point of use with a ferro-fluid that reveals the pattern, and erased by the user in a degausser, thereby providing evidence of magnetic field exposure greater than the coercivity of the material in the visual indication device. The sensitivity of a fusible device or film can be made directional, for example, by being made more resistant to magnetic field strength in the direction of the film than through it. A range of films having different coercivities can be offered. Such sensors can be used in sets of three, each applied to an orthogonal surface of a media sample. Although such fused devices or films provide a “go” or “no go” indication of whether a sensed field reached a particular threshold, or at best, a low resolution comparable to a bar graph indicator, such devices cannot provide a measurement of the actual field strength provided by the degausser.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.